transistor

TRANSISTOR
 Transistor is a
semiconductor device
made of a solid piece of
semiconductor material
,with atleast three
terminals for connection
to an external circuit
TYPES
 Semiconductor device-germanium ,silicon, gallium
arsenide ,silicon carbide etc.
 Structure – BJT ,JFET ,IGFET(MOSFET) ,IGBT.
 Polarity – NPN ,PNP(BJT’S); N-Channel,Pchannel(FET’S).
 Maximum power rating – Low ,Medium ,High
APPLICATIONS
TRANSISTOR :
As an switch
As an amplifier
As an oscillator
TRANSISTOR AS A SWITCH
When the switch is in open position, there
will be no potential difference between the
base and the emitter.
•The depeltion region across the emitter-base
junction is large.
•No current flows through it.
•The transistor is off.
TRANSISTOR AS A SWITCH
When the transistor is off:
 The emitter -collector terminal acts like an open.
 The supply of 10 volts drops across the collector
output.
In this condition, the transistor is reffered to as
being in cut-off.
When the switch is in open position, a voltage is
present across base-emitter junction.
•The depletion region across the emitter-base junction
shrinks as much as possible.
•Thee transistor turn on as hard it can.
•Maximum current flows through it.
TRANSISTOR AS A SWITCH
In this condition transistor is reffered to as being in
saturation.
 The collector-emitter terminal acts like a closed
switch.
 0 volts drop across the output, causing 10 volts to
drop across the load resistor.
An amplifier is a device which is used for increasing
the amplitude of variation of alternating voltage or
current or power.
The amplifier thus produces an enlarged version of
the input signal.
There are two input terminals for the signal to be
amplified and two output terminals for the
connecting the load and a means of supplying power
to the amplifier.
From mobile phones to televisions, vast number of
products include amplifiers for sound reproduction,
radio transmission, and signal processing .The first
discrete transistor audio amplifiers barely supplied a few
hundred milliwats, but power and audio fidelity
gradually increased as better transistor became available
and amplifier architecture evolved.
Transistors are commonly used in modern musical
instrument amplifiers, where circuit upto a few hundred
watts are common and relatively cheap. It has largely
replaced valves in instrument amplifiers.
The BASIC TRANSISTOR AMPLIFIER amplifies by
producing a large change in collector current for a
small change in base current. This action results in
voltage amplification because the load resistor placed
in series with the collector reacts to this large changes
in collector current which, in turn , results in large
variation in the output voltage.
CLASS OF AMPLIFIERS
The class of amplifier
operation
is
determined by the
portion of the input
signal for which
there is an output
Class ‘A’ amplifiers : These are biased so that variation in input
signal polarities occur within the limits of cut-off and saturation
.Biasing an amplifier in this manner allows collector current to
flow during the complete cycle(360 degrees) of the input signal,
thus providing an output which is the replica of the input but
180
degrees
out
of
phase.
Class ‘AB’ amplifier : These are biased so that collector current
is zero (cutoff) for a portion of one alternation of the input
signal. These collector current will flow for more than 180
degrees but less than 360 degrees of the input signal. The class
‘AB’ amplifier is commonly used as a push-pull amplifier to
overcome a side effect of class B operations.
Class ‘B’ amplifier : These are biased so that so that collector
current is cut off during one half of the input signal. Thus, for a
class B operation, collector current will flow for approximately
180
degrees(half)
of
the
input
signal.
The class ‘B’ operated amplifier is used as an audio amplifier
and as the driver-and power amplifier stage of transmitters.
Class ‘C’ amplifier : These are biased so that collector current
flows for less than one half cycle of the input signal.
The Class ‘C’ amplifier is used as a radio frequency amplifier in
transmitters.
TRANSISTOR
AS
AN
OSCILLATOR
A simple L-C circuit can be used to produce
electrical oscillation of desired frequency. The
radio waves which are used as carrier waves in
radio communications are produced by this circuit
called oscillators.
Tank circuit consisting of an inductance L and a
capacitor C, connected in parallel is the simplest
type of electrical oscillating system. In this circuit
electrical energy once given to the circuit,
oscillates as magnetic energy in the inductance
and electrostatic energy in the capacitance .
Due to internal resistance of the inductance coil etc.
there occurs a small but constant energy loss and
hence the oscillation produced are damped.
The damped waves are suitable for the transmission of
code messages but to transmit speech or music, we
require undamped electronic waves or carrier waves.
The same can be produced by transistor as an oscillator.
TRANSISTOR AS AN OSCILLATOR
The advantages that allowed transistors to replace
vaccum tubes are :
•Small size and minimul weight
•Less power consumption
•More rugged
•Lower voltages
•Less heat
•Greater reliability
The “first generation” of computers used vaccum tubes
which generated large amounts of heat and were bulky
and unreliable.?The development of the transistor was
the key to computer miniaturization and reliability.
The “second generation” of the computers, through
the late 1950’s and 1960’s featured boards filled with
individual transistors and magnetic memory cores.
Subsequently transistor, other components and their
necessary wiring were integrated into a simple, mass
manufactured component : the integrated circuit.
Transistors incorporated into integrated circuit.
Transistors incorporated into IC have replaced most
discrete transistors in modern digital computers.